1. Field of the Invention
The present invention relates generally to computer hard disk drives, and more particularly to systems and methods for determining which of two coil current values (estimated and saturated) to use in a head position model.
2. Description of the Related Art
Computer hard disk drives include rotatable disks on which data is stored in concentric data tracks. Data is transferred between the disks and a computer that is external to the hard disk drive via read and write heads within the hard disk drive. The heads are closely spaced from the disks, and are attached to actuator arms which in turn are moved by a voice coil motor (VCM) to move the heads radially relative to the disks. With this structure, as the disks rotate past the heads, the heads can be positioned over an appropriate data track.
Electrical current is supplied to the VCM by a power amplifier. The amount of current supplied depends on the distance through which the heads must be moved. This distance is known to a microprocessor inside the hard disk drive; consequently, the current that is to be requested from the power amplifier is determined by the microprocessor.
It will be appreciated that the microprocessor must precisely control the positioning of the heads to facilitate rapid and accurate head placement, to thereby optimize the rate at which data is transferred to and from the hard disk drive. Accordingly, the movement of the heads must be modelled, to enable the microprocessor to derive an accurate estimate of the position of the heads. From the above discussion, it will be further appreciated that the modelling process accounts for the current supplied to the VCM, because the operation of the VCM and, hence, the position of the heads, depends on the amount of current supplied to the VCM.
It happens that the modelling process cannot simply assume that the requested current is the current actually supplied to the VCM by the power amplifier. This is because the power amplifier can be driven into saturation, in which case the amplifier will output only a saturation current which is less than the requested current. Recognizing this, existing hard disk drive systems use two models for predicting head placement, namely, a linear model that is intended to be used when the power amplifier is not saturated, and a saturated model to be used otherwise. Essentially, the saturation model calculates the saturation current from nominal values for power amplifier voltage, coil resistance, and head velocity, and replaces the requested current input to the linear model with the estimated saturation current.
Accordingly, it is important to know when the power amplifier is saturated, so that the appropriate model can be used. The saturation point of a power amplifier, however, is not necessarily constant. Instead, it can vary with such factors as temperature. Thus, the precise saturation point of the power amplifier ideally is periodically determined, so that the correct model of head movement can be used.
One way to determine precisely when a power amplifier is saturated is to directly measure its output current, digitize it, and then compare it to the requested current value. If the actual current is less than the requested current, the power amplifier is saturated. Unfortunately, this requires that relatively expensive dedicated analog-to-digital converters (ADCs) be incorporated into the hard disk drive.
Recognizing the above problem, U.S. Pat. No. 4,914,644, owned by the present assignee, discloses a method for determining when the power amplifier is saturated based on the difference between the requested head velocity and the estimated head velocity, without requiring dedicated ADCs to do so. While effective for its intended purpose, it happens that the '644 model might not detect unexpected power amplifier saturation, particularly during head deceleration. The present invention, however, recognizes that it is possible to provide an accurate and precise determination of when a power amplifier is saturated, and to provide information to the head position model to improve is predictive abilities. This information can take the form of an estimate of the current actually supplied to the voice coil, or simply as knowledge that the requested current is larger than the available current.
Accordingly, it is an object of the present invention to provide a system for determining when a power amplifier in a computer hard disk drive is saturated. Another object of the present invention is to provide a system for improving the predictive ability of a head position model in a hard disk drive. Yet another object of the present invention is to provide a cost-effective, easy to use system for determining when a power amplifier in a hard disk drive is saturated.